The modern data network space is made up of a plurality of Autonomous Systems (ASs) that are directly or indirectly linked to a communications network, such as the public internet. In this respect, it will be noted that the classical definition of an “autonomous system” refers to a set of one or more routers under a single technical administration, using an Interior Gateway Protocol (IGP) and common metrics to route packets within the autonomous system, and using an Exterior Gateway Protocol (EGP) to route packets to other autonomous systems. Since this classic definition was developed, it has become common for single autonomous systems to use several interior gateway protocols and sometimes several different sets of metrics within the AS. In the present application, the term Autonomous System (AS) is used to emphasize the fact that, even when multiple IGPs and metrics are used, the technical administration of an AS appears to other autonomous systems to have a single coherent interior routing plan and presents a consistent picture of what destinations are reachable through it.
FIG. 1 is a block diagram showing a typical autonomous system 2 having three areas 4a-c (Area 0.0.0.1, Area 0.0.0.2 and Area 0.0.0.3) that are linked to a backbone network 6 via two Area Border Routers (ABRs) 8 and to a communications network 10 such as the public internet via an Autonomous System Border Router (ASBR) 12. Each area 4 includes one or more Internal Routers (IRs) 14, which control the forwarding of traffic among user machines 16 (e.g. client PCs and content servers) and respective ABRs 8 hosting the area 4. Each of the routers 8,14 are coupled together via links 18 (which may be physical or logical links) through which packetized data is forwarded.
The topology of the autonomous system 2 illustrated in FIG. 1 is typical of that set up within an enterprise or campus Local Area Network (LAN) to connect various domains (e.g. departmental LANs) represented by each area 4 to the communications network 10. Traffic forwarding external to the autonomous system 2 (both to and from the autonomous system 2), is controlled by the ASBR 12 using an Exterior Gateway Protocol (EGP) such as Border Gateway Protocol (BGP) in a manner known in the art. Within the autonomous system 2, traffic forwarding is controlled using an Interior Gateway Protocol (IGP) such as Open Shortest Path First (OSPF) protocol.
Using this arrangement, information concerning addresses located outside the autonomous system 2, and reachable through the communications network 10, can be obtained using BGP messages received by the ASBR 12. BGP route information received in this manner is checked against predetermined OSPF policies, which control the generation of Type-5 (and/or Type-7, if the autonomous system 2 is an NSSA area) Link State Advertisement (LSA) messages by the ASBR 12. The BGP route information is then propagated through the autonomous system 2 by flooding the Type-5 (or Type-7) LSAs into the autonomous system 2, such that each router 8,12,14 in the autonomous system 2 obtains the BGP route information, and can write appropriate entries into its respective forwarding table (not shown).
Typically, information concerning addresses within an area in the autonomous system 2 is propagated throughout the autonomous system 2 by flooding Type-3 LSAs into the autonomous system 2 from the ABR 8 hosting the involved address. This enables each router 8,12,14 in the autonomous system 2 to obtain the internal route information, and write appropriate entries into its respective forwarding table.
As is well known in the art, the routing of traffic within the autonomous system 2 is controlled by the forwarding table maintained by each router, which maps packets received by a router 8,12,14 to downstream links 18 connected to the router, typically on the basis of the contents of the destination address field of the traffic header. Exemplary data fields within the forwarding table include: IP Address; Mask; Next Hop and Next Hop Interface. As each packet arrives at a router, its destination address is read and used to query the forwarding table. If a matching route in the forwarding table is located, the corresponding Next Hop and Next Hop Interface fields are used to forward the packet to a downstream link towards its destination. Otherwise, the packet is discarded.
The routes identified in a conventional forwarding table are always “inclusionary”, in the sense that a router can forward packets to any route (or address) identified in the forwarding table. Conversely, the router is unable to forward packets to any routes (or addresses) that are not identified in the forwarding table. Typically, the forwarding table contains a list of explicitly defined routes to which packets may be forwarded, and/or a default route to which the router can forward packets that do not match any of the explicitly defined routes.
Co-pending and co-assigned U.S. patent application Ser. No. 09/662,108, filed on Sep. 14, 2000, and entitled “Exclusion Routes in Border Gateway Protocol” (BGP), teaches a method of controlling traffic within a BGP network by means of “exclusion” routes, which can be entered into the forwarding table in a conventional manner, but which explicitly define routes to which traffic may not be forwarded. This modifies the effect of default routes, thereby allowing control over traffic flows within the network, while at the same time maximizing performance by minimizing the size of the forwarding table maintained by each BGP router. For example, an exclusion route can be defined in the forwarding table of the ASBR 12 (which is a BGP router) such that packets originating in the autonomous system 2 and destined for one or more “restricted” addresses in the communications network 10 are discarded by the ASBR 12. Similarly, exclusion routes may be defined such that packets originating in the communications network 10 and destined for selected addresses in the autonomous system 2 are discarded by the ASBR 12.
The use of explicitly defined exclusion routes, as described in U.S. patent application Ser. No. 09/662,108, provides enhanced control over BGP traffic, and thus can be used for engineering and policing of traffic entering and leaving the autonomous system 2.
A limitation of the arrangement of U.S. patent application Ser. No. 09/662,108 is that the implementation of policy-based traffic forwarding by means of BGP exclusion routes affects the entire autonomous system equally. In many instances, it is desirable to implement different policy-based traffic forwarding regimes (e.g. providing different levels of access and security) in different areas of an autonomous system. For example, an enterprise may wish to partition its enterprise LAN into discrete areas, each having respective different levels of security and public access. In the Autonomous system illustrated in FIG. 1, for example, Area 0.0.0.1 4a may be used to provide secure space for employees, and Area 0.0.0.2 4b used by accounting and corporate finance departments. Both of these areas 4a,4b must therefore be carefully protected against unauthorized access. On the other hand, Area 0.0.0.3 4c may be used for distribution of product information, and handling customer inquiries and product orders, and therefore must be readily accessible from the communications network 10. It is desirable for internal routers 14 located in Areas 0.0.0.1 and 0.0.0.2 4a and 4b, respectively, to obtain route information concerning addresses within Area 0.0.0.3 4c, in order to enable maintenance and other administrative functions. However, in order to maintain security, it is important that internal routers 14 within Area 0.0.0.3 4c be unable to access addresses within Areas 0.0.0.1 and 0.0.0.2 4a and 4b. 
One method of accomplishing this is to manually configure the respective forwarding tables of ABR(A) 8a and ABR(B) 8b to include only explicitly defined inclusion routes to which traffic may be forwarded. However OSPF normally operates to advertise new and/or changed addresses throughout the autonomous system by flooding LSAs from the router hosting the new addresses. Thus manually configuring the respective forwarding tables of ABR(A) 8a and ABR(B) 8b with explicitly defined routes requires that the conventional route-learning functionality of OSPF be defeated. This creates scalability and network maintenance difficulties as the configuration of the autonomous system changes.
Request for Comments (rfc)-2740 describes OSPF for IP version 6, which attempts to overcome some of the limitations of autonomous system-wide propagation of LSAs, by allowing a router originating the LSA to restrict propagation of the LSA to a link, a local area, or the entire autonomous system. However, this functionality cannot accommodate a situation in which it is desired to selectively propagate an LSA into some areas of the autonomous system, but not others. For example, the autonomous system of FIG. 1 contains three areas 4, and it is desired that LSAs originating in Area 0.0.0.1, be propagated to Area 0.0.0.2 to enable nodes in Area 0.0.0.2 to access addresses in Area 0.0.0.1. However, it is important that these same LSAs be prevented from propagating into Area 0.0.0.3, and so prevent unauthorized access to addresses in Area 0.0.0.1 from the (publicly accessible) Area 0.0.0.3.
Accordingly, a method and apparatus for enabling flexible control of traffic forwarding within an OSPF network, while ensuring a high level of security and scalability, remains highly desirable.